1. The Field of the Invention.
The present disclosure relates generally to light modulation devices, and more particularly, but not necessarily entirely, to methods of calibrating light modulation devices.
2. Description of Background Art
A wide variety of devices exist for modulating a beam of incident light. Light modulating devices may be suitable for use in displaying images. One type of light modulating device, known as a grating light modulator, includes a plurality of reflective and deformable ribbons suspended over a substrate. The ribbons are parallel to one another and are arranged in rows and may be deflected, i.e., pulled down, by applying a bias voltage between the ribbons and the substrate. A first group of ribbons may comprise alternate rows of the ribbons. The ribbons of the first group may be collectively driven by a single digital-to-analog controller (“DAC”) such that a common bias voltage may be applied to each of them at the same time. For this reason, the ribbons of the first group are sometimes referred to herein as “bias ribbons.” A second group of ribbons may comprise those alternate rows of ribbons that are not part of the first group. Each of the ribbons of the second group may be individually controllable by its own dedicated DAC such that a variable bias voltage may be independently applied to each of them. For this reason, the ribbons of the second group are sometimes referred to herein as “active ribbons.”
The bias and active ribbons may be sub-divided into separately controllable picture elements referred to herein as “pixels.” Each pixel contains, at a minimum, a bias ribbon and an adjacent active ribbon. When the reflective surfaces of the bias and active ribbons of a pixel are co-planar, essentially all of the incident light directed onto the pixel is reflected. By blocking the reflected light from a pixel, a dark spot is produced on the display. When the reflective surfaces of the bias and active ribbons of a pixel are not in the same plane, incident light is diffracted off of the ribbons. Unblocked, this diffracted light produces a bright spot on the display. The intensity of the light produced on a display by a pixel may be controlled by varying the separation between the reflective surfaces of its active and bias ribbons. Typically, this is accomplished by varying the voltage applied to the active ribbon while holding the bias ribbon at a common bias voltage.
The contrast ratio of a pixel is the ratio of the luminosity of the brightest output of the pixel and the darkest output of the pixel. It has been previously determined that the maximum light intensity output for a pixel will occur in a diffraction based system when the distance between the reflective surfaces its active and bias ribbons is λ/4, where λ is the wavelength of the light incident on the pixel. The minimum light intensity output for a pixel will occur when the reflective surfaces of its active and bias ribbons are co-planar. Intermediate light intensities may be output from the pixel by varying the separation between the reflected surfaces of the active and bias ribbons between co-planar and λ/4. Additional information regarding the operation of grating light modulators is disclosed in U.S. Pat. Nos. 5,661,592, 5,982,553, and 5,841,579, which are all hereby incorporated by reference herein in their entireties.
As previously mentioned, all of the bias ribbons are commonly controlled by a single DAC and each of the active ribbons is individually controlled by its own dedicated DAC. Each DAC applies an output voltage to its controlled ribbon or ribbons in response to an input signal. Ideally, each DAC would apply the same output voltage in response to the same input signal. However, in practice, it is very difficult to perfectly match the gain and offset of all the DACs to the degree of accuracy that is required for optimum operation of a light modulator due to the differences in the individual operating characteristics of each DAC. Thus, disadvantageously, the same input values may not always result in the same output for different DACs. This discrepancy means that two active ribbons whose DACs receive the same input signal may be undesirably deflected in different amounts thereby making it difficult to display an image with the proper light intensities.
In view of the foregoing, it is understood that prior to use the combination of DACs and ribbons on a light modulating device must be calibrated to ensure that the desired light intensities are correctly reproduced in a displayed image. As mentioned, calibration is required due to the fact that the offset voltage and gain of each DAC may be different. Thus, given the same DAC input values for the active ribbons of two pixels, the displayed light intensities generated by the two pixels will likely be different because the active ribbons will be deflected in different amounts. Calibration is intended to ensure that the different operational characteristics of the DACs and ribbons are taken into account during operation of the light modulation device.
The calibration process may be divided into two separate calibration processes, namely, a dark-state calibration and a bright-state calibration. Generally speaking, the dark-state calibration is an attempt to determine the DAC input values at which the pixels produce the minimum amount of light possible and the bright-state calibration is an attempt to ensure that each pixel produces the same light intensity for the same source input values.
Prior to the present disclosure, known calibration techniques for light modulation devices did not always produce the best possible results. In particular, previously known dark-state calibration methods involved calibrating all of the pixels on a light modulating device at the same time using a group-calibration process. For example, using one previously available dark-state calibration process, all of the DACs for the active ribbons of a light modulation device were first set with an input value of 0. (However, due to the offset of each of the active ribbons' DAC, a small voltage of about 0.5 volts was actually applied to the active ribbons thereby pulling them slightly down.) Then, the input value to the single DAC controlling all of the bias ribbons was experimentally varied until the best overall dark state for all of the pixels was determined by visual inspection from a human. As a result of the above described group-calibration process for the dark state, the constituent ribbons of some of the pixels were not necessarily co-planar as is required for the minimum light intensity output. Thus, some of the pixels still produced some light output even when they were set to a dark state.
The previously available bright-state calibration processes used a brute force method to determine the correct input value for a DAC based upon a desired intensity level. In particular, the previous bright-state calibration methods used an 8-entry look-up-table (“LUT”) to store the DAC input value to use for each individual pixel (DAC values were interpolated for intensities in between). The desired DAC value for each of the 8 LUT intensities was found by performing a binary search on DAC values until the desired intensity was reached. This search was performed on each pixel for each of the 8 LUT entries. One drawback to this method is that it took over 8 hours to calibrate a light modulation device with just 1000 pixels.
In view of the foregoing, it would therefore be an improvement over the previously available calibration methods to provide a dark-state calibration that minimizes the light output of each pixel individually instead of on a collective basis. It would further be an improvement over the previously available dark-state calibration methods to provide an alternative to using visual inspection by a human to determine a minimum light intensity output. It would further be an improvement over the previously available bright-state calibration methods to provide a bright-state calibration method that is quicker and easier to implement for a light modulating device with a high number of pixels.
The features and advantages of the disclosure will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by the practice of the disclosure without undue experimentation. The features and advantages of the disclosure may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims.